The present invention relates to a valve apparatus and more particularly to a valve apparatus suitable for use in a base valve of a tube type hydraulic shock absorber, for example, in which a disk-shaped valve member is secured to a valve body by staking.
A dual tube hydraulic shock absorber attached to a suspension system of an automobile is disclosed, for example, in Japanese Patent Application Publication No. 2003-254374. The hydraulic shock absorber has a dual tube structure comprising a cylinder and an outer tube provided to surround the outer periphery of the cylinder. An annular reservoir is formed between the cylinder and the outer tube. A piston is slidably fitted in the cylinder. The piston has a piston rod connected thereto to form a piston assembly. A base valve is provided at the lower end of the cylinder to divide between the interior of the cylinder and the reservoir. The cylinder has a hydraulic fluid sealed therein. The reservoir has the hydraulic fluid and gas sealed therein. The piston assembly and the base valve are provided with damping force generating mechanisms each including an orifice, a disk valve, etc. The flow of hydraulic fluid induced by the sliding movement of the piston in the cylinder is controlled by the damping force generating mechanisms, thereby generating damping force. In addition, the hydraulic fluid is exchanged between the cylinder and the reservoir through the base valve, thereby compensating for a volumetric change in the cylinder due to the piston rod entry into and withdrawal from the cylinder.
An example of the structure of a base valve of such a dual tube hydraulic shock absorber will be explained below with reference to FIG. 10. As shown in FIG. 10, a base valve has a valve body 2 that is secured to the lower end of a cylinder (not shown) to divide between a cylinder chamber C and a reservoir R. The valve body 2 has extension hydraulic fluid passages 3 and compression hydraulic fluid passages 4 axially extending therethrough for communication between the cylinder chamber C and the reservoir R. A check valve 5 (disk valve) is secured to the upper end of the valve body 2 to allow only the flow of hydraulic fluid through the extension hydraulic fluid passages 3 from the reservoir R to the cylinder chamber C. A disk valve 6 is secured to the lower end of the valve body 2 to control the flow of hydraulic fluid through the compression hydraulic fluid passages 4 from the cylinder chamber C to the reservoir R, thereby generating damping force.
The check valve 5 and the disk valve 6 are secured, together with retainers 7 and 8 and washers 9 and 10, to the valve body 2 with a pin 12 inserted through an opening 11 in the center of the valve body 2. The pin 12 has at the lower end thereof a flange portion 13 that abuts against the washer 10. The pin 12 is inserted through the valve body 2, the check valve 5, the disk valve 6, the retainers 7 and 8 and the washers 9 and 10, and a distal end portion thereof is staked to form a staked portion 14, thereby securing together these members as one unit.
The distal end of the pin 12 can be deformed to form the staked portion 14, for example, by an orbital (Taumel) staking process wherein a tilted punch is pressed against the distal end of the pin 12 while being revolved on a predetermined orbit, thereby forming a rivet head continuously. Thus, it is possible to form the staked portion 14 efficiently with a low load while preventing frictional heat generation.
The following problems, however, are experienced with the valve apparatus of the above-described related art in which the check valve 5, the disk valve 6 and so forth are secured to the valve body 2 by staking the pin 12. When the staked portion 14 is formed at the distal end of the pin 12, as shown in FIG. 11, radial strains occur near the staked portion 14, causing the pin 12 to slightly increase in diameter. Consequently, the shank of the pin 12 has its diameter increased from the original diameter A as follows. A portion of the shank of the pin 12 nearest to the staked portion 14 has the largest diameter B. The diameter gradually decreases as the distance from the staked portion 14 increases, but the diameter C of a portion of the pin shank where the check valve 5 is fitted is still slightly larger than the original diameter A.
Accordingly, the inner diameter of the check valve 5 is forcedly enlarged by the enlarged diameter shank of the pin 12, and this induces wavy deformation on the surface of the check valve 5. The deformation causes the valve opening characteristics of the check valve 5 to become unstable and also degrades sealing performance, resulting in the hydraulic shock absorber failing to provide stable damping force. FIG. 8A shows a measuring result of the amount of deformation D at the outer peripheral portion of the check valve 5. It will be understood from FIG. 8A that there are two peaks of deformation D at two diametrically opposing portions of the check valve 5, and this causes the sealing performance to be degraded. Curve (A) in FIG. 9 shows the rate of change of pressure in the upper chamber (cylinder chamber C) with respect to the number of times of pressurization when the upper chamber and the lower chamber (reservoir R) divided by the base valve were repeatedly pressurized at a pressure corresponding to a piston speed of 0.1 m/sec. It can be seen from curve (A) in FIG. 9 that there is a pressure change of about 19% after 200 times of pressurization, i.e. the change of valve opening characteristics with time is large, and thus the valve characteristics are unstable.